Experimental study of monazite/melt partitioning with implications for the REE, Th and U geochemistry of crustal rocks

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• 作者：Aleksandr S. Stepanov ; ^{stepanovas@gmail.com} ; Joerg Hermann ; Daniela Rubatto ; Robert P. Rapp
• 关键词：Monazite ; REE ; Trace element ; Fractionation
• 刊名：Chemical Geology
• 出版年：2012
• 期刊代码：12_00092541
• 类别：ear
• 出版时间：18 March, 2012
• 卷：300-301
• 期：Complete
• 页码：200-220
• 文件大小：1798 K

摘要

We report the results of monazite/melt partitioning experiments conducted in the piston-cylinder apparatus at 10-50 kbar and 750-1200 掳C, using a synthetic granite mix with approximately 10 wt.%H₂O and doped with trace-elements in proportions corresponding to the composition of monazite. Monazite was produced in all experiments, generally in the form of small grains. Electron microprobe and laser ablation-ICP-MS analyses were carried out on the resulting 鈥渕onazite-melt鈥?mixes from these experiments, and the composition of the crystallized monazite calculated using regression analysis.

Where H₂O is in weight percent, T is in Kelvin, P in kbar and 鈭?#xA0;LREE is the sum of La-Sm in ppm; X_mnz^LREE is the molar ratio of LREE to the sum of all cations (REE, Th, U) in monazite.

REE, Th, U, Y, V and As partition into monazite, whereas other trace elements (Li, Be, B, Sc, Ti, Mn, Sr, Zr, Nb, Ba, Hf, Ta and Pb) have monazite/melt partition coefficients less than unity. Monazite shows the greatest preference for LREE from La to Nd, with a progressive decrease in partition coefficients for Sm and the HREE. The partition coefficients for Th are 30%higher than those for the LREE, and Th/LREE ratios are independent of pressure and temperature. Partition coefficients for U are 4-23 times lower than for the LREE.

The new experimental data provide a numerical basis for modeling the behavior of LREE, Th and U during fractional crystallization of granitic magmas, as well as the melting in the presence of monazite, both within the continental crust, and in subduction zones.